Higher order interaction inhibits bacterial invasion of a producer-predator microbial community*

Microbial communities must resist invasion in order to maintain biodiversity, stabilize industrial bioreactors, and preserve human microbiome health. It is widely believed that the more diverse a microbial community is, the more resistant to invasion it will be, and that this increased invasion resistance arises from a niche complementarity effect: more diverse communities consume a greater range of resources and thus eliminate niches for would-be invaders. Here we show that in a community of the algae Chlamydomonas reinhardtii (producer) and the ciliate Tetrahymena thermophila (predator), invasions by the bacteria Escherichia coli fail even when there is a niche of resources available to the bacteria. In contrast, bacteria successfully invade communities of algae or ciliates alone. We attribute the invasion resistance of the algae-ciliate community to a higher-order (3-way) interaction: the algae inhibits the bacteria’s ability to aggregate which leaves the bacteria vulnerable to the ciliate’s predation. This method of invasion resistance requires both the algae and the ciliate to be present and thus provides an example of diversity leading to invasion resistance due to a higher-order interaction rather than niche complementarity.